What's the difference between setting up Mediawiki by adding it from AWS marketplace vs. creating a server on Bitnami and then adding my AWS credentials to the Bitnami management console? Is there a difference and if there is, what set up is easier to do for someone with little experience in cloud computing?

The main difference is that if you use Bitnami Cloud Hosting, you will be able to monitor your server with the monitor tab, also, you will be able to create server backups in only two click (and restore those backups in two cllicks too).

Also, with Bitnami Management Console you will be able to launch not only MediaWiki, but other applications during the server creation process. Once the server is created, you will be able to add your own domain, and configure mediawiki in the desired suburi or subdomain.

You could do all this stuff with the AWS Marketplace Cloud Image installing stacks manually, and if you want to configure your own domain you only could do it manually and modifying files in the server, so if you are not used to do it you might spend some time on it.

As you want to use only a Mediawiki installation, you could use our Developer plan with the AWS Free tier, so you wouldn't be charged during one year if you only use one server (if you create Backups you will be charged). Take a look at: https://bitnami.com/cloud